1. Field of the Invention
The present invention relates to an image signal generating method and apparatus for generating an image signal by taking an image including a repeated pattern, e.g., a radiation image including a grid pattern corresponding to a grid used when the radiation image is taken. The invention also relates to a program for causing a computer to execute such an image signal generating method.
2. Description of the Prior Art
Radiation recording and reproduction systems have been known from Japanese Unexamined Patent Publication Nos. 55(1980)-12429, 56(1981)-11395, 55(1980)-163472, 56(1981)-164645 and 55(1980)-116340, for example. These known systems use a storage-type phosphor (stimulable phosphor) for the imaging of objects such as a human body. When exposed to radiation such as X-rays, xcex1-rays, xcex2-rays, xcex3-rays, electron beams and ultraviolet rays, the storage-type phosphor stores part of the radiation. Upon irradiation with a stimulating ray such as visible light, the phosphor emits an amount of stimulated light in proportion to the amount of energy stored. Such known systems with the storage-type phosphor first take a radiation image of an object such as a human body and store it on the storage-type phosphor in the form of a sheet. The storage-type phosphor sheet is then scanned by a stimulating ray such as a laser beam in a main-scan direction while the sheet is transported in a sub-scan direction, thereby producing a stimulated emission of light. The stimulated light emission is photo-electrically read by a reading means such as a photomultiplier to produce an image signal. Based on this image signal, the radiation image of the object can be output as a visible image on a recording medium such as a photosensitive material or on a CRT.
When taking and recording the radiation image of the object on a recording sheet such as the above-described storage-type phosphor sheet, a grid is sometimes disposed between the object and the sheet. The grid is formed of bars of a radiation-impermeable material, such as lead, and a radiation-permeable material, such as aluminum or wood, which are alternately located at small pitches of about 4.0 bars/mm. The grid functions to prevent the sheet from being irradiated by the radiation scattered by the object. Such use of the grid helps to reduce the amount of radiation scattered by the object falling on the sheet, thereby improving the contrast of the radiation image of the object. On the other hand, when the image including an image of the grid is either increased or reduced in size, aliasing occurs due to folding of the frequency characteristics curve, depending on the ratio of enlargement. Furthermore, if such aliasing corresponds with the spatial frequency of the grid pattern, for example, moirxc3xa9 fringes will arise, which will further degrade the quality of the reproduced image.
To counter this problem, it has been proposed to perform filtering processing to remove the spatial frequency components of the grid pattern, so that the moirxc3xa9 or aliasing can be reduced and therefore an image can be obtained that is easy to observe (U.S. Pat. No. 5,028,784, for example). According to this method, the aliasing in the grid pattern caused by folding is removed by filtering processing when the spatial frequency of the grid pattern is higher than a spatial frequency (Nyquist frequency) required for image information. For example, if the pitch of the grid is 4.0 bars/mm and the Nyquist frequency is 2.5 cycles/mm, aliasing will arise at 1 cycle/mm. Accordingly, the radiation image is first read at smaller sampling intervals than the sampling intervals necessary for image information, and the obtained image data is subjected to a filtering process which removes the spatial frequencies near 4 cycles/mm. Thereafter sampling is effected at sampling intervals required for image information, thereby removing the aliasing.
Since the grid pattern is included in the image signal as a rectangular signal, harmonic components of the grid pattern are contained in a region of high frequencies which are integral multiples of the spatial frequency corresponding to the grid pattern. For example, as shown in FIG. 8, if the grid pattern has a spatial frequency of 4 cycles/mm, a first harmonic component is generated at twice that, i.e., at 8 cycles/mm, and a second harmonic component is generated at three times that grid pattern spatial frequency, i.e., at 12 cycles/mm. When the Nyquist frequency or the spatial frequency necessary for image information is 5 cycles/mm, there will be no moirxc3xa9 in the spatial frequency components corresponding to the grid pattern. However, the first harmonic component folds back at 5 cycles/mm to cause aliasing at 2 cycles/mm, and the second harmonic component folds back at 5 cycles/mm and 0 cycle/mm to cause aliasing at 2 cycles/mm. As a result, a moirxc3xa9 appears at 2 cycles/mm.
According to the above-mentioned method known from U.S. Pat. No. 5,028,784, the harmonic components cannot be removed because of the filtering processing for the removal of the spatial frequency of the grid pattern. As a result, the above-mentioned moirxc3xa9 at 2 cycles/mm cannot be removed. Further, additional aliasing or moirxc3xa9 will be generated if the image containing the aliasing or moirxc3xa9 is increased or reduced in size, thereby adversely affecting the examination of the object. Aliasing due to high-frequency components appears in a relatively low-frequency band within the image. However, such low-frequency band also contains much useful information for the image. Accordingly, an attempt to remove the aliasing may also result in removal of information useful for the image.
In view of the foregoing problems of the prior art, it is an object of the invention to provide an image signal generating method and apparatus by which a radiation image having no aliasing or moirxc3xa9 can be obtained even if the spatial frequency of the grid pattern is smaller than the spatial frequency necessary for image information, and a program for causing a computer to execute such an image signal generating method.
The image signal generating method according to the present invention comprises the steps of:
obtaining an initial image signal by reading an original image including a repeated pattern repeated with a lower spatial frequency than a maximum spatial frequency of a desired spatial frequency band, wherein the reading is performed at sampling intervals corresponding to a spatial frequency which is not smaller than n (n=a positive number of 2 or more) times the spatial frequency of the repeated pattern;
filtering the initial image signal using a filter for removing spatial frequencies corresponding to harmonics components of the repeated pattern; and
obtaining an image signal representing the original image by sampling the thus filtering-processed initial image signal at predetermined sampling intervals corresponding to the maximum spatial frequency or a Nyquist frequency.
In a preferred embodiment of the image signal generating method according to the invention, a sub-sampled signal is obtained by sub-sampling the filtered initial image signal at smaller sampling intervals than the predetermined sampling intervals, and the filtering and sub-sampling of the sub-sampled image signal are repeated until the image signal sampled at the predetermined sampling intervals is obtained.
In a further preferred embodiment of the image signal generating method, a further filtering processing is performed on the image signal to remove the spatial frequency of the repeated pattern.
The maximum spatial frequency of the desired spatial frequency band refers to a spatial frequency necessary for image information, i.e., a Nyquist frequency which is determined by the sampling intervals used in reproducing the image signal representing the original image.
The original image may be a radiation image taken with the use of a grid. In that case, the repeated pattern included in the original image is a grid pattern corresponding to the applied grid. Also in that case, the original image (i.e., the radiation image) is read out from a recording sheet such as a storage-type phosphor sheet. However, the original image is not limited to the radiation image, but may be an image of any other type as far as it includes a repeated pattern such as a stripe pattern or a wire-netting pattern.
When the image signal is obtained from the recording sheet, the recording sheet is scanned by a stimulating beam. With regard to the main-scan direction, the obtained signal tends to be blurred and have poor sharpness due to the emission response delay or the like of the stimulated light, so that the harmonics components do not easily arise. In addition, during the read-out, the signal is obtained as a continuous analog signal as far as the main direction is concerned, so that the harmonics components can be removed by an analog filter. On the other hand, in the sub-scan direction, there is no emission response delay in the stimulated light emission and the harmonics components cannot be removed by an analog filter. As a result, in the sub-scan direction, harmonics components of the repeated pattern (or the grid pattern) are generated and aliasing results due to folding during the sampling. Accordingly, in the case where the original image (or the radiation image) is read out from the recording sheet, the invention has only to effect a filtering processing at least with respect to the sub-scan direction.
The image signal generating apparatus according to the invention comprises:
reading means for obtaining an initial image signal by reading an original image including a repeated pattern repeated with a lower spatial frequency than a maximum spatial frequency of a desired spatial frequency band, wherein the reading is performed at sampling intervals corresponding to a spatial frequency which is not smaller than n (n=a positive number of 2 or more) times the spatial frequency of the repeated pattern;
filtering means for performing a filtering processing on the initial image signal using a filter for removing spatial frequencies corresponding to harmonics components of the repeated pattern; and
sampling means for obtaining an image signal representing the original image by sampling the filtering-processed initial image signal at predetermined sampling intervals corresponding to the maximum spatial frequency or a Nyquist frequency.
In a preferred embodiment of the image signal generating apparatus according to the invention, the sampling means obtains a sub-sampled image signal by sub-sampling the filtering-processed initial image signal at smaller sampling intervals than the predetermined sampling intervals, and the filtering processing and sub-sampling of the sub-sampled image signal are repeated until an image signal sampled at the predetermined sampling intervals is obtained.
In another preferred embodiment of the image signal generating signal generating apparatus according to the invention, the filtering means effects a further filtering processing on the image signal to remove the spatial frequency of the repeated pattern.
In addition, a program may be provided for causing a computer to execute the image signal generating method according to the present invention.
Thus, in accordance with the invention, when the spatial frequency of the repeated pattern is lower than the maximum spatial frequency of the desired spatial frequency band, there is first obtained the initial image signal by reading the original image at the sampling intervals that correspond with the spatial frequency which is n or more times the spatial frequency of the repeated pattern. As a result, the initial image signal contains harmonics components of the repeated pattern. The initial image signal is then filtering-processed by a filter, whereby spatial frequencies corresponding to the harmonics components of the repeated pattern are removed. The thus filtering-processed initial image signal is sampled at predetermined sampling intervals corresponding to the maximum spatial frequency or a Nyquist frequency, whereby the image signal representing the original image is obtained. Since the spatial frequencies corresponding to the harmonics components of the repeated pattern are thus removed, it is possible to obtain an image signal for reproducing an image having no aliasing or moirxc3xa9 due to the folding back of the harmonics of the repeated pattern even if the sampling is performed at the predetermined sampling intervals.
When n is relatively large, the harmonics components of the repeated pattern will appear in a plurality of frequency bands. On the other hand, in order to remove those harmonics components caused in the multiple frequency bands, the filter must be increased in size. It is difficult, however, to design such a large-sized filter as required, and also the filtering processing will require a longer period of time. In the present invention, therefore, the need for a large-sized filter is eliminated and the image signal can be obtained by a simple calculation by repeating the filtering processing and sub-sampling until an image signal with predetermined sampling intervals is obtained.
In a preferred embodiment of the apparatus according to the invention, a further filtering processing is performed on the obtained image signal to remove the spatial frequency of the repeated pattern. This prevents the generation of aliasing or moirxc3xa9 caused by the repeated pattern no matter how much the image signal is increased or reduced in size. Thus, it becomes possible to obtain an image signal which can be used to reproduce a high-quality image with a desired ratio of enlargement. In particular, in the case where the original image is a radiation image for medical use, doctors etc. may conduct more accurate diagnosis by obtaining an image signal for reproducing an image free from aliasing or moirxc3xa9.